Hypothesis

We hypothesize that an age‑related shift in the gut microbiota toward a higher acetate‑to‑butyrate ratio drives increased release of microglial extracellular vesicles (EVs) containing cytosolic DNA, which amplifies systemic inflammaging and accelerates neurodegeneration. Conversely, restoring a lower acetate‑to‑butyrate ratio suppresses EV biogenesis, reduces neuroinflammatory priming, and improves cognitive outcomes.

Mechanistic Rationale

Gut‑derived metabolites modulate microglial EV formation through two opposing pathways:

• Butyrate inhibits histone deacetylases (HDACs), leading to reduced expression of neutral sphingomyelinase 2 (nSMase2), a key enzyme for EV budding; this limits EV release.
• Acetate activates the G‑protein‑coupled receptor GPR43 on microglia, triggering intracellular calcium flux that promotes nSMase2 activation and EV shedding.

In aging, dysbiotic communities often produce excess acetate and depleted butyrate, tilting the balance toward EV‑mediated DNA release. These EVs carry mitochondrial and nuclear DNA that act as damage‑associated molecular patterns (DAMPs), stimulating TLR9 in peripheral myeloid cells and further increasing blood‑brain barrier permeability. This creates a feed‑forward loop where gut dysbiosis fuels microglial EV production, EV‑derived DNA fuels peripheral inflammation, and peripheral cytokines worsen gut barrier integrity.

Evidence supports each link: butyrate’s HDAC inhibitory effect on microglial activation is documented [1]; GPR43‑dependent calcium signaling regulates vesicular trafficking in immune cells [2]; microglial EVs bearing cytosolic DNA propagate neurotoxicity [3]; and fecal microbiota transplantation normalizes metabolite profiles and reduces pathology [5].

Testable Predictions

1. Older adults with a higher fecal acetate‑to‑butyrate ratio will exhibit elevated plasma levels of microglial EV‑associated DNA (measured by CD63+/DNA+ vesicles).
2. Experimental elevation of butyrate (e.g., via resistant starch supplementation) will decrease microglial EV‑DNA concentrations and improve MoCA scores over 12 weeks.
3. Pharmacological inhibition of nSMase2 (using GW4869) will abolish the covariance between acetate‑to‑butyrate ratio and EV‑DNA, confirming mechanistic specificity.

Experimental Design

• Cohort: 120 participants aged 65‑80, stratified by baseline amyloid‑PET status.
• Intervention arms: (a) high‑fiber resistant starch (30 g/day) to boost butyrate; (b) iso‑caloric low‑fiber control; (c) resistant starch + GW4869 (nSMase2 inhibitor) in a double‑blind, placebo‑controlled subset.
• Measurements: (i) fecal SCFA profiling (GC‑MS) at weeks 0, 6, 12; (ii) plasma microglial EV‑DNA (flow cytometry for CD63+/dsDNA+); (iii) serum cytokines (IL‑6, TNF‑α, LPS‑binding protein); (iv) cognitive battery (MoCA, episodic memory); (v) optional PET‑MRI for amyloid and microglial activation (TSPO).
• Analysis: Mixed‑effects models testing interaction between time, intervention, and acetate‑to‑butyrate ratio on EV‑DNA and cognitive change. Mediation analysis to assess whether EV‑DNA mediates the effect of SCFA ratio on cognition.

Potential Outcomes and Falsifiability

• Confirmation: A significant reduction in EV‑DNA and cognitive improvement in the resistant‑starch arm, correlated with increased fecal butyrate and decreased acetate‑to‑butyrate ratio, would support the hypothesis. The loss of this effect in the GW4869 subgroup would demonstrate that EV release is necessary for the observed benefits.
• Falsification: If resistant starch alters SCFA ratios but does not change microglial EV‑DNA or cognitive performance, or if EV‑DNA levels remain unchanged despite robust SCFA shifts, the hypothesized mechanistic link would be refuted. Similarly, if nSMase2 inhibition does not affect the relationship between SCFA ratio and EV‑DNA, alternative EV biogenesis pathways would need consideration.

By targeting a measurable metabolite ratio and its direct consequence on microglial EV release, this framework transforms the "messy" gut‑brain axis into a quantifiable, intervenable axis for mitigating inflammaging‑related cognitive decline.